There are many cases in which it is desirable to know if people and/or animals are present inside a vehicle. One such case is to prevent children, incapacitated people or animals from being forgotten inside a vehicle. Every year numerous cases occur where children, people or animals are forgotten in vehicles for extended periods. In warm climates this may lead to hospitalization due to dehydration or even to death.
Another type of case is at border checkpoints where vehicles are inspected to prevent unauthorized people from entering a protected area, crossing over from one state to another or to prevent smuggling animals or people into a state. Typically the unauthorized people may be intentionally hidden inside the vehicle to prevent them from being detected or they may have stowed away in the vehicle with the driver unaware of their presence.
Various systems have been suggested to detect the presence of people by placing seismic sensitive sensors (e.g. geophones) inside the vehicle (to prevent forgetting children) or in contact with the vehicle externally (e.g. at a checkpoint) to sense low level vibrations caused by the person's cardiorespiratory system. Typically the person's heart-beat, breathing, blood-flow and muscle reflections form vibrations having a frequency of between 1-20 Hz, which can be detected to identify the presence of a person in the vehicle. Typically a seismic sensor will measure motion/vibrations of the body of the vehicle relative to the ground via the tires, which serve as springs holding the mass of the vehicle. The seismic sensors provide an electrical signal that can be recorded and/or analyzed by a computer or electronic circuit to identify animate presence.
Standard analysis methods generally analyze the measured signal quantitatively without attempting to identify specific characteristics and potentially distinguishing features, which would indicate if the motion is due to an animate entity or to environmental factors (e.g. wind or rain). An example of such an analysis includes identifying zero crossings in the identified frequency range or summating motion energy based on the measured signal.
However one problem is that the measurements of the sensor are generally not taken in a sterile environment but rather are affected by the vibrations caused by environmental noise such as wind gusts, rain drops and other vehicles passing by. The environmental factors can lead to a high percentage of false identification of animate presence.
One solution to the problem of environmental interference is by using additional sensors to measure the ground vibrations next to the vehicle and/or vibrations in the air/environment surrounding the vehicle. By comparison of the measurements of the various sensors the environmental influence can be subtracted.
An alternative method that relies only on sensors measuring the vibrations of the vehicle attempts to enhance analysis of the signal by integrating the energy of the signal to determine its relative strength and comparing the results to a threshold value to determine if extra energy is available from the hidden person or animal. One problem with this method is that it takes a relatively long time (e.g. a few minutes) to collect a large enough sample to get reliable results and it still suffers from extra energy provided by external vibrations.